1. Field of The Invention
This invention relates to a method and an apparatus for precisely specifying a sound field at various locations within a medium (e.g., a human body or an acoustical cavity), through the independent manipulation of inputs to a synthesizing array of reciprocal transducers. The invention can be used, for example, to destroy calculi, while simultaneously specifying a safe level of sound intensity in the area immediately surrounding the calculi, as well as in the intermediary tissue between the transducers and the calculi. In addition, objects and fluids can be acoustically levitated in microgravity or other environments through the utilization of this same principle.
2. Description of the Prior Art
In the prior art, it is known to use a large number of piezoelectric elements arranged in a mosaic pattern on a spherical surface so that ultrasonic waves transmitted from the elements converge on a single point. This is shown, for example, in U.S. Pat. No. 4,907,573 issued to Nagasaki. With reference to FIG. 1 of the present invention, there is shown such an arrangement of transducers. The piezoelectric elements 1 collectively constitute a transducer 2 which is applied to a subject 5 through a liquid bag 4 filled with an ultrasonic transmission medium 3, such as water, so that ultrasonic shock waves generated by the piezoelectric elements converge, for example, on a diseased portion existing in a kidney 6 or a calculus 7 to be treated and to thereby break it up. The water is used as an acoustic transmission medium in order that the ultrasonic shock waves do not attenuate during its propagation.
The ultrasonic shock wave is generated by applying a driving impulse voltage to the piezoelectric elements from a driving pulse generator 8. A mechanical-scanning type ultrasonic probe 9 and an ultrasonic diagnosis device 10 coupled to the probe may be provided for the purposes of locating the calculus.
Conventional non-invasive medical lithotripsy methods generally utilize either a focused array of transducers, each emanating sound amplitude levels (required for calculi destruction) that are extremely large, and/or a method for focusing an array of transducers, each emanating a relatively smaller sound amplitude level, such that large sound amplitude levels occur only in a region near the calculi to be destroyed.
For example, U.S. Pat. No. 4,526,168 issued to Hassler et al. discloses an apparatus for destroying calculi in body cavities. Hassler uses an ultrasonic transmitter 13 coupled with a number of time delay devices (7-11) to adjust the distance of focus so as to dispose the focus at the location of the calculus 6 of a kidney 5. Hassler, therefore, uses the same input to drive all of his transducers and focus all of his transducers at a single point.
U.S. Pat. No. 4,907,573 issued to Nagasaki discloses an ultrasonic lithotresis apparatus. This apparatus includes a driver circuit 8 for driving each of a plurality of transducers which are focused to converge at a single point. Delay lines 14 are adapted to finely shift the phases of the drive pulses from pulser 13 so that the phases of the transducers coincide with each other at the point of convergence.
Similar apparati are disclosed in U.S. Pat. No. 4,823,773 issued to Naser et al. and U.S. Pat. No. 4,858,597 issued to Kurtze et al.
A drawback of the prior art is that each transducer element within the array is driven by the same input signal, either in phase or time delayed with respect to the others. Additionally, the medium between each transducer and the intended focus is assumed to be homogeneous for each transducer. These factors are reasons why the resulting shock wave pulse is not very effective and why numerous repetitions of the pulse are needed for successful destruction of a calculus. Moreover, since the region of convergence of this resultant, large amplitude sound field cannot be precisely guaranteed, but only insinuated, i.e., since an isotropic, homogeneous medium is assumed to represent the actual anisotropic, inhomogeneous medium, and since each source is driven by the same input signal, these methods may result in the bruising and damaging of tissue and to cavitation, causing free-radical production, as well as pain to the victim, all of which are undesirable.